1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a power transmission belt having longitudinally spaced driving/driven teeth.
2. Background Art
It is known to use toothed belts with a pitch between 8.0mm and 9.25 mm to drive overhead cam shafts on vehicles. Recent developments in the automotive industry have resulted in higher engine output in a more compact engine compartment. As a result, toothed belts on these engines have been required to operate under high load, at high tension, and at high temperatures. This environment demands a high quality toothed belt capable of withstanding these severe conditions.
One attempt to improve conventional toothed belts to withstand these severe conditions has been to improve the materials from which the toothed belts are made. Instead of using conventional chloroprene rubber to define the teeth and an outside backing layer, hydrogenated nitrile rubber (HNBR), chlorosulfonated polyethylene (CSM), and the like have been utilized. The material of the load carrying cords has been changed from conventional E-glass to high strength glass and aramid fiber.
One example of a toothed belt using hydrogenated nitrile rubber and K-glass cords is disclosed in Unexamined Japanese Utility Model Publication 111848/1989. An example of a toothed belt using hydrogenated nitrile rubber aramid fiber cords is disclosed in Unexamined Japanese Patent Publication No. 215186/1993.
It is also known to change the composition of a fabric cover layer that is placed over the teeth on the inside of the belt body. One modification is to use aramid fiber in place of conventional 6 nylon and 6-6 nylon. An example of a toothed belt using aramid fiber in a fabric cover layer is disclosed in Unexamined Japanese Patent Publication No. 8948/1992.
While the above changes in the tooth and backing rubber, fabric cover layer, and load carrying cords has significantly improved heat resistance, abrasion resistance and resistance to chipping, these changes have resulted in other limitations and drawbacks.
The previously mentioned tooth pitch of 8.0 mm to 9.25 mm is measured along a hypothetical pitch line during running of a belt under a predetermined tension. The hypothetical pitch line is a hypothetical distance where a distance (PLD value) from the bottom land face to the pitch line is set at 0.686 ram, with the teeth of the belt in mesh with grooves on a cooperating pulley. It is a common practice in designing the cross section of toothed belts to make the distance between the bottom, inwardly facing land face between adjacent teeth and the center of the load carrying cords at 0.686 mm with the belt in a running state. The distance between the bottom land surface and the center of the cord is then between 0.68 and 0.72 mm with the belt in a stationary state.
With the above type of belt running, the land surface touches or is forced against the tip of a tooth on a cooperating pulley. This induces friction and causes abrasion between the land surface and the pulley tip. Additionally, frictional heat is generated which may degrade or cause failure of the load carrying cords. As the distance between the land surface and the load carrying cord is reduced, the detrimental effect caused by contact between the pulley tooth and land surface increases to the point that considerable degradation of the tensile strength of the load carrying cords may occur. This may ultimately result in belt failure.